The basic principle of an atomic clock is to use as a frequency reference a transition of an atom, ion or molecule that naturally has metrological qualities and that is accessible. To do this it is necessary to use a rigorously fixed frequency possessing a character that is absolute both in time and space.
Atomic transitions are considered to have this absolute character of invariance in time and in space. This postulate has not been invalidated up to now. Caesium has been chosen to define the second.
Optically pumped caesium-beam atomic clocks are well known and have demonstrated their reliability.
Optically pumped atomic clocks generally comprise optically pumped caesium tubes, or, in other words, optically pumped atomic resonators using caesium, that comprise many elements inside a vacuum-tight envelope.
FIG. 1 schematically shows an atomic resonator 1 of the prior art, and more particularly its vacuum-tight enveloped portion 2, which is equipped with optical interfaces 3 and comprises a caesium oven 11 and a magnetic shield 8 that is passed through by a caesium beam. The magnetic shield 8 surrounds a resonant cavity 4, caesium traps 6 made of graphite, a magnetic field coil 9, and an RF cable 10.
The caesium oven 11 of the vacuum-tight envelope 2 is placed outside the magnetic shield 8, as is the interface 11 a for connection with the caesium oven 11. A vacuum pump 7 is placed partially outside the vacuum-tight envelope 2 and partially inside, as an optical unit comprising a laser source 5a and optical mirrors 5b. 
Such resonators comprise about 250 elements, which, for reasons of assembly and performance constraints, are integrated into the interior of the vacuum chamber or envelope. Many of these elements have a function that does not require operation under vacuum, but they are placed under vacuum for reasons of manufacturing constraints; these elements represent a mass of more than 5 kg.